1. Field
This patent specification relates to a method, an apparatus and a computer program product for performing a shading correction of an image processing apparatus, and more particularly adjusting shading correction data for an abnormal pixel using a predetermined value to perform an appropriate shading correction.
2. Discussion of the Background
Generally, an image reading apparatus includes an image sensor, such as a CCD, and an analog/digital converter to convert optical information obtained by optically scanning an original document into an electric signal using the image sensor. The electric signal is then converted into multivalued image data by the analog/digital converters.
In such an image reading apparatus, the conversion process to the image data is controlled according to a contrast of an original document and a luminance change in a light source. Because an output from the image sensor is especially important, a shading correction is performed to correct a sensitivity of the image sensor.
A white plate is used for a shading correction. A standard value for the shading correction is set based on a result of reading the white plate.
Therefore, if the white plate is dirty, a standard value may not be correct. Thus, improper shading correction may be performed resulting in output of degraded image data. Further, if fouling adheres to a platen which contacts a reading surface of an original document or to a component in a light path, such as a lens, or when the image sensor is defective, degraded image data can be output, which is different from the output when the white plate is dirty because not only standard shading correction data but also read image data are affected. Such a degradation of image data caused by the above-described fouling cannot be removed by a shading correction.
Japanese Patent Laid-Open Publication No. 11-122490 discusses a technology for handling degradation caused by dirt or fouling to improve a reading performance of an image reading apparatus.
According to Japanese Patent Laid-Open Publication No. 11-122490, the image reading apparatus, in which a shading correction on image data of an original document is performed using shading correction data obtained by reading the white plate, includes an abnormal pixel detection section which detects an abnormal pixel in the image data obtained by reading the white plate. The apparatus also includes a diagnose section to determine whether or not the abnormal pixel detected by the abnormal pixel detection section is caused by dirt on the white plate.
The apparatus further includes an abnormal pixel correction section to correct shading correction data obtained at a position where an abnormal pixel is detected when the detected abnormal pixel is caused by dirt of the white plate. The abnormal pixel correction section corrects the shading correction data by using data of a pixel around the abnormal pixel when the detected abnormal pixel is caused by factors other than dirt on the white plate.
A memory device is required to store information about a position of an abnormal pixel to identify the abnormal pixel in the shading correction data in the image reading apparatus discussed in Japanese Patent Laid-Open Publication No. 11-122490. Standard shading data or image data must be corrected based on information on the position of the abnormal pixel.
For example, a special memory device may be employed to store a result of the determination made by the abnormal pixel determination section so as to store information about the position of the abnormal pixel.
However, an abnormal pixel may be very rarely detected when the apparatus is used in a normal condition, and entropy may be very small from an information engineering standpoint. Nevertheless, it increases in costs to provide a special memory device to store image information for such corrections in order to form a high-quality image.